Currently there are zero viruses affecting Mac OS X in public circulation, but there are other kinds of malware existing, that can infect your Mac.
To learn more about malware in Mac OS X and what steps can be taken to protect yourself, read the following F.A.Q.:

I want a very basic antivirus that isn't gonna eat up my system resources.

As others have said, ClamXav (which is free) is one of the best choices, since it isn't a resource hog, detects both Mac and Windows malware and doesn't run with elevated privileges. You can run scans when you choose, rather than leaving it running all the time, slowing your system. ClamXav has a Sentry feature which, if enabled, will use significant system resources to constantly scan. Disable the Sentry feature. You don't need it. Also, when you first install ClamXav, as with many antivirus apps, it may perform an initial full system scan, which will consume resources. Once the initial scan is complete, periodic on-demand scans will have much lower demands on resources.

Disable Java in your browser (Safari, Chrome, Firefox). This will protect you from malware that exploits Java in your browser, including the recent Flashback trojan. Leave Java disabled until you visit a trusted site that requires it, then re-enable only for the duration of your visit to that site. (This is not to be confused with JavaScript, which you should leave enabled.)

For added security, make sure all network, email, financial and other important passwords are long and complex, including upper and lower case letters, numbers and special characters.

Always keep your Mac and application software updated. Use Software Update for your Mac software. For other software, it's safer to get updates from the developer's site or from the menu item "Check for updates", rather than installing from any notification window that pops up while you're surfing the web.

That's all you need to do to keep your Mac completely free of any Mac OS X malware that has ever been released into the wild.

Yes, you're absolutely right about iAntivirus! For some reason I thought iAntivirus was another OS X port of ClamAV. I just now noticed that it's a Norton product, who I've been burned by (on Windows) numerous times.

The MAS version of VirusBarrier Pro is excellent. I use it because I send/receive lots of files to/from Windows machines and it is quite effective. Intego's other, "realtime" VirusBarrier X5 is absolute ****, since it needlessly consumes system resources, slowing down the system and running the battery down. Also, it's 100% useless at detecting the sparse OS X malware.

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"The reason I talk to myself is that I am the only one whose answers I accept." - George Carlin

There are no demonstrated methods to bypass the runtime security mitigations in Mac OS X Lion/ML. For example, Safari on a Mac running Lion wasn't compromised at the most recent pwn2own. So, malware that installs via remote exploitation is not an issue.

Mac OS X Lion/ML include Gatekeeper. Set Gatekeeper to only allow apps from the Mac App Store. This will prevent malicious apps inadvertently installed from the web by users from executing. Make sure to verify that an app is safe prior to manually bypassing the Gatekeeper restrictions.

Java has been exploited via logical errors allowing the Java sandbox to be bypassed when malicious Java applets are run from the browser. Mac OS X Lion/ML don't include Java by default and the most recent Apple provided Java no longer includes browser plugins. So, malware that installs via Java isn't an issue unless the user installs Java from Oracle. Disable Java in the browser via the Safari preferences if Java from Oracle is installed.

This basically covers all vectors in which malware is installed.

If an antivirus solution is required, install ClamXav from the Mac App Store and run weekly scans. Also, make sure to manually scan any documents sent to Windows users that have been received from potentially untrustworthy sources. Manually scanning documents is performed by secondary clicking the item to be scanned and selecting "Scan with ClamXav" from the drop down menu.